Development of transition metal-catalyzed carboxylation reactions is an important topic not only from the standpoint of scientific issue for using carbon dioxide, a less reactive molecule, in catalytic reactions, but also from that of global issue for utilizing carbon dioxide as an abundant carbon resource. Dr. Nobuharu Iwasawa has realized several new catalytic carbon dioxide fixation reactions based on the design and construction of transition metal complexes and their systems. His major achievements are summarized below.
1. Design and utilization of transition metal complexes containing multidentate ligands for carboxylation reactions
By using PSiP-pincer palladium complexes, catalytic hydrocarboxylation reactions of allenes and 1,3-dienes were realized. The success of the reaction is the generation of nucleophilic σ-allylpalladium intermediate instead of electrophilic π-allylpalladium owing to the structural restriction of the PSiP-pincer complex. Furthermore, the isolation and X-ray crystallographic analysis of Si-H coordinated palladium(0) complex was achieved for the first time, and detailed analyses of the reaction mechanism were carried out by isolation and identification of the key intermediates together with reaction kinetic analyses and theoretical approach, resulting in clarification of the dynamic behavior of the complexes derived from the strained nature of the PSiP-pincer complex.
This chemistry was further developed into generation of a new type of complexes containing a metal-metal bond by utilizing 6,6"-bisphosphinoterpyridine ligand (PNNNP-type ligand) as a scaffold. Successive introduction of two kinds of metals into the ligand enabled generation of various kinds of complexes containing a metal-metal bond, and the Al-Pd complex showed remarkable catalytic activity for hydrosilylation of carbon dioxide.
2. Direct C-H carboxylation reactions using rhodium and palladium complexes
Direct C-H carboxylation reactions are ideal from the viewpoint of the atom economy. By utilizing the C-H activation ability and high nucleophilicity of rhodium(I) complexes, a catalytic system was realized for direct C-H carboxylation of aromatic and olefinic C-H bonds with carbon dioxide. The reaction was further developed into catalytic benzoic acid synthesis from benzene and CO2. These results are important as a method for generating highly nucleophilic transition metal species by C-H activation. Another direct C-H carboxylation was also established by using a palladium(II) catalyst, which enabled preparation of synthetically useful coumarin derivatives from ο-alkenylphenols.
3. Nickel and ruthenium-catalyzed synthesis of acrylic acid
Acrylic acid is an industrially important compound and efficient preparation of acrylic acid from ethylene and carbon dioxide would bring innovation to its production. A robust nickel catalyst was designed and prepared by using an NHC ligand containing a phosphine side arm and the highest TON was achieved for nickel-catalyzed acrylic acid synthesis from ethylene and carbon dioxide. This reaction was further developed into a catalytic methylmalonate salt synthesis from ethylene and CO2. Furthermore, ruthenium(0) complexes bearing a tetradentate phosphine ligand were utilized for the acrylate synthesis for the first time as a ruthenium complex.
4. Establishment of dual catalyst systems for carboxylation reactions using visible light irradiation
In the transition metal-catalyzed carboxylation reactions, it is often necessary to regenerate reactive metal hydrides or low valent metals from the corresponding metal carboxylates. To realize this using visible-light energy, dual catalyst systems composed of carboxylation and photoredox catalysts were developed. By using rhodium and ruthenium dual catalyst system, hydrocarboxylation reaction of styrenes was realized. By also using palladium and iridium dual catalyst system, carboxylation of aryl halides and related substrates was achieved. In these reactions, amines are employed as an electron source under visible-light irradiation instead of conventionally employed stoichiometric metallic reductants, and these reactions are the first successful results of utilizing visible-light for regeneration of reactive carboxylation catalysts.
As a further development of reduction method, hydrocarboxylation of styrenes using H2 and CO2 was achieved under irradiation by using rhodium and ruthenium dual catalyst system. Use of Davephos was essential for the desired hydrocarboxyation reaction suppressing the undesired hydrogenation, and a perfect atom-economical reaction was realized.
In summary, Dr. Iwasawa has developed various transition metal-catalyzed carboxylation reactions based on the design of transition metal complexes and their systems and has opened new avenues for advancing the frontier of organic chemistry, transition metal chemistry, and photochemistry. Therefore, his achievements were recognized as worthy of the Chemical Society of Japan Award.